What's the difference between birds that get killed by cars, and those that don't?

The dead ones tend to have smaller brains, scientists who performed 3,521 avian autopsies said Wednesday.

What might be called the "bird brain rule" applies to different species, depending on the ratio of grey matter to body mass, they reported in the journal Royal Society Open Science.

Crows, for example, have big brains relative to their size, and have shown a remarkable knack for navigating oncoming traffic.

While picking at road kill on Florida highways, earlier research showed, the scavengers learned to ignore cars and trucks whizzing by them within inches, but would fly away just in time when threatened by a vehicle in their lane.

"I don't know if we can say they are 'smarter,' but they do exhibit cognitive behaviour that makes them likely to survive," lead author Anders Pape Moller, an evolutionary biologist at the French National Center for Scientific Research, told AFP.

Pigeons, by contrast, appear to be less adept at avoiding collisions, a deficiency visible in compressed form on most big-city streets.

More surprisingly, the relation between brain size and traffic accidents also holds within the same species, the study found.

"Common European blackbirds, house sparrows and robins all show this difference in individuals that were hit by cars, and those that were not," Moller said by phone.

Undamaged brains from birds killed in accidents - weighed to within a hundredth of a gram - were consistently smaller, he said.

Grim tallyIn total, the researchers examined 251 different species.

The finding raises the question of whether certain species, and certain populations within species, have evolved over the space of dozens of generations such that birds with car-dodging abilities have become more common.

Moller is skeptical.

Recent research has shown that evolutionary changes in many animals - fish and insects, in particular - are occurring in response to climate change and human encroachment on habitat far more quickly that scientists imagined possible.

But given that fast-moving cars have only been on the roads for less than a century, it is unlikely that birds have adapted - as a species level - that fast.

"The other problem is that total mortality due to traffic is not big enough to cause evolutionary change," Moller said.